1. Field of the Invention
The invention relates to DC voltages, and more particularly to measurement of DC voltages.
2. Description of the Related Art
A portable electronic device generally uses a battery power supply. Batteries provide the portable electronic device with DC voltages. The DC voltage level provided by batteries reduces with time. When the DC voltage reduces to a threshold level, the portable electronic device cannot obtain enough power for normal operation, thereby making the portable electronic device nonoperational. To maintain normal operation of portable electronic devices, the portable electronic devices are generally equipped with a DC voltage meter for monitoring the DC voltage level supplied by batteries. Thus, when the DC voltage level reduces to a threshold level, a user of the portable electronic device will be alerted to the low threshold level by the DC voltage meter and can thus change the batteries of the portable electronic device.
A majority of portable electronic devices are digital devices. A DC voltage meter measuring a DC voltage for a digital portable device therefore converts a measurement value of the DC voltage to a digital value with an analog-to-digital converter, and then outputs the digital value to the digital portable device. When the digital portable device receives the digital value from the DC voltage meter, the digital portable device can then convert the digital value to a picture and show the picture on a screen of the digital portable device to indicate the DC voltage level of a battery. The analog-to-digital converter, however, is limited to precision level thereof and often, the digital value converted from the measurement value cannot accurately reflect the DC voltage level of the battery. For example, a 5-bit analog-to-digital converter has 32 (=25) output values. When the 5-bit analog-to-digital converter has an input voltage range of 0V˜3V, the precision level of the analog-to-digital converter is therefore 0.09V (=3V/32). Thus, voltage variation with a level less than the precision level of the analog-to-digital converter cannot be detected by the analog-to-digital converter.
Referring to FIG. 1, a schematic diagram of a conventional method for DC voltage measurement is shown. Assume that an analog-to-digital converter of a DC voltage meter has five output values of 0, 1, 2, 3, and 4. The output values respectively correspond to voltage ranges of −0.5V˜0.5V, 0.5V˜1.5V, 1.5V˜2.5V, 2.5V˜3.5V, 3.5V˜4.5V. When an input DC voltage is 2.8V which falls within the voltage range of 2.5V˜3.5V, the analog-to-digital converter converts the input DC voltage to a digital output value of 3. Even if the DC voltage meter continues to measure the input DC voltage at time t1, t2, t3, t4, and t5, the analog-to-digital converter only outputs 5 output values of 3. The user therefore only obtains an approximate measurement voltage of 3V, and cannot obtain a precise measurement voltage of 2.8V. The approximate measurement voltage may lead to inaccurate determination of the battery voltage level, and the user may not be notified in time to change batteries of their portable electronic device. A method for precisely measuring a DC voltage is therefore required.